A substantial number of materials begin at or pass through a sticky, or cohesive, phase during drying operations. These materials can include food products, polymers, pharmaceuticals and waste streams, among others. Polymer hydrogels are one class of materials that can experience a cohesive phase during drying. Polymer hydrogels are characterized, in part, by their ability to absorb water and retain it under pressure. Polymer hydrogels are widely used in the manufacture of personal hygiene products, but they also have important new pharmaceutical applications.
Many pharmaceutically important materials, such as polymer hydrogels, are initially produced with a higher moisture content than is found in the end product. Accordingly, one of the final steps in the manufacture of such products can include drying the product to an acceptable moisture level. Drying is usually necessary because it helps to minimize transportation and packaging costs. Drying may also stabilize the product against microbial or chemical degradation. Such materials are usually dried in any of a number of commercially available dryers, which typically subject the material to continuous agitation as it dries. The available dryers can vary in the method by which they reduce moisture and by the materials that they can process.
Many polymeric hydrogels experience a cohesive phase at certain levels of moisture content. During this phase, polymer particles adhere with high affinity to each other and to equipment surfaces. This can make drying polymer hydrogels particularly challenging. In dryers that use mechanical agitation, hydrogels in a cohesive phase can cause strain upon and damage to impellers, turbines, and end seal assemblies. The increased torque caused by agitating a material in its cohesive phase can damage or stress the motors and drive systems used to agitate the material. The motors and gearboxes of such driers need to be suitably robust which is reflected in capital and operating costs.
Alternatively, attempts have been made to avoid a cohesive phase. Dry product can be back mixed to lower the moisture content of the dryer feed below the threshold required for the cohesive phase. However, product back mixing can be undesirable in that the size of equipment required is correspondingly increased. Further, product back-mixing is unsuitable for materials which are cohesive at very low moisture content or for materials that are very wet at the beginning of the drying step. Product back-mixing may also be undesirable for the preparation of materials that must meet exact production standards, such as materials for which loss of batch integrity is unacceptable.
Other methods to avoid a cohesive phase use additives to assist in drying, such as azeotrope-forming solvents or agents that affect the surface wetting of the product. However, the use of additives and organic solvents can be detrimental to pharmaceutical purity and generally increases production costs.
A need exists for an improved method to reduce the moisture content of materials that experience a cohesive phase. Manufacturers need a process that will permit processing of these materials without the higher capital, operating and maintenance expenses that can result from practicing conventional drying techniques. Additionally, many manufacturers, such as those in the pharmaceutical industry, need a process that does not use unnecessary additives or solvents and maintains high purity standards.
This invention relates to a method for drying a material, such as a polymer hydrogel, that experiences a cohesive phase at certain levels of temperature and moisture content. This invention is based, in part, on the discovery that a material can be dried, in the absence of agitation, by applying a vacuum to the material as it passes through a cohesive phase. Thus, the invention allows for the suspension of agitation while a composition passes through its cohesive phase.
The method of the present invention comprises agitating a composition comprising a solid and a liquid while removing the liquid until the solids content of the composition reaches a level at which the composition enters a cohesive phase, halting agitation, removing liquid from the composition in the absence of agitation, and resuming agitation.
Consequently, practice of this invention can eliminate the problems associated with adhesion of the material to itself and to process equipment during the cohesive phase. Practice of the present invention can avoid the need for product back-mixing, for the use of azeotrope-forming solvents, or for the adding of agents that effect the surface wetting of the product. By avoiding these conventional drying techniques, the method of the present invention can help pharmaceutical manufacturers to maintain high purity standards and also to ensure batch integrity. Additionally, since agitation is suspended during the cohesive phase, practice of the present invention can avoid stress, strain, and damage to dryers that use mechanical agitation.